QString str = QString("C:/Qt/lei-muban.dot").arg(dir.absolutePath());

This line of code creates a QString object named "str" that contains a path to a file named "lei-muban.dot". The path is specified as "C:/Qt/lei-muban.dot", and the ".arg()" method is used to insert the absolute path of the directory specified by the "dir" object.

For example, if "dir.absolutePath()" returns "C:/users/johndoe/documents/", the resulting string stored in "str" would be "C:/Qt/lei-muban.dot/C:/users/johndoe/documents/".

Note that this may not be a valid file path, as it could result in a directory being treated as part of the filename. It would be more appropriate to use QString's "append()" method to add the directory path to the end of the file path, like so:

QString str = QString("C:/Qt/lei-muban.dot").append(dir.absolutePath());

This would result in "str" containing the string "C:/Qt/lei-muban.dot/C:/users/johndoe/documents/" (using the example directory path from earlier).